This application claims benefit to European Patent Application No. EP99 122 975.8, filed Nov. 11, 1999.
1. Field of the Invention
The present invention relates to a fire alarm, and more particularly to a multiple or a multi-sensor fire alarm with an optical module, a combustion gas sensor, a temperature sensor and an electronic evaluator.
2. Description of the Related Art
Modern fire alarms, in particular multi-sensor or multiple fire alarms, can detect fires with a high degree of reliability, and they are very sensitive. In fire alarms of this type, an optical module is used to detect smoke and a temperature sensor is used for detection of the heat occurring at an outlet of a fire. The optical module can measure either the light from the light source that is scattered by smoke particles (scattered-light alarm), or the light from the light source that is attenuated by these smoke particles (a point-extinction or transmitted-light alarm). In both cases, the optical module is designed so that the interfering external light cannot penetrate the measuring chamber while smoke can easily do so. For example, a scattered light alarm with a temperature sensor is disclosed in EP-A-0 654 770. The temperature sensor is used both for increasing the sensitivity and for improving the functioning of the scattered light alarm.
However, the high sensitivity can sometimes lead to false alarms, which is undesirable for a number of reasons. For example, false alarms tend to reduce the attentiveness of the relevant safety personnel. In addition, the fire service and/or the police demand payment for call-outs caused by false alarms which can rise progressively with the number of false alarms. Accordingly, an improved fire alarm with an arrangement for protection against false alarms is required.
An object of the present fire alarm is to improve the false alarm protection and to reduce the fire alarm""s response time.
Another object of the present invention is to provide a more homogeneous alarm response characteristic which would allow the alarm to respond in substantially the same way to different fires, i.e., not extremely rapidly to one type of fire and extremely slowly to another, or even not at all.
In a first embodiment, the fire alarm includes an optical module, a temperature sensor and at least one additional sensor for detecting a combustion gas. The electronic evaluator is coupled to the optical module, temperature sensor and combustion gas sensor and diagnoses various types of fire based on the signals from the sensors.
The optical module of the fire alarm, which generally includes a light source, a measuring chamber and an optical receiver, can be designed so that either the light from the light source that is scattered by smoke particles or the light from the light source that is attenuated by these smoke particles is measured in the measuring chamber. In the first case the detection principle is that of a scattered-light alarm and in the second case that of a transmitted-light alarm. Here the scattered-light alarm can be designed as a forward-scatter or back-scatter device or as a forward-scatter and back-scatter device. The latter has the advantage that the type of smoke that is present can be ascertained with the aid of the scatter at different scatter angles. In this regard, see WO-A-84 01650, which is hereby incorporated by reference.
In one embodiment of the fire alarm, the electronic evaluator is a fuzzy controller.
In another embodiment of the fire alarm, at least one of the included combustion gas sensors is a carbon monoxide sensor.
In yet another embodiment of the fire alarm, the light source of the optical module is designed to emit radiation in the wavelength range of visible light. In this case, the wavelength of the radiation emitted by the light source can be in the range of blue or red light and is preferably 460 nm and 660 nm, respectively.
In a further embodiment of the fire alarm, at least one polarization filter is provided in the path between the light source and the optical receiver. The polarization filter can take the form of an active polarizer with an electrically-adjustable polarization plane.
A type of problem diagnosis in which the fuzzy controller monitors whether certain faults frequently occur below the respective alarm thresholds is also possible. The fuzzy controller can report such faults to the control center or the operating personnel via a suitable communications interface and in this way indicate potential sources of interference whose cause may possibly lie in an incorrect application of the relevant alarm.
Preferably, the active polarizer is formed by a liquid crystal display whose polarization plane can be adjusted by applying a voltage.